Method and system for network setup and maintenance and medium access control for a wireless sensor network

ABSTRACT

A sensor network having a first command center and a first access node, comprising a wireless transceiver, coupled to the command center. The sensor network may also include a plurality of nodes individually comprising a wireless transceiver and a directional antenna, wherein each of the plurality of nodes is successively located in a downlink direction relative to the first access node, and is configured to wirelessly communicate via the directional antenna with at least one node of a first neighbor group in a first direction and at least one node of a second neighbor group in a second direction. In addition, a sensor device is individually coupled to at least one of the nodes, and is configured to provide sensor data for the first command center.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/088,313, filed Aug. 12, 2008, which is incorporated in its entiretyherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government may have rights in this invention pursuantto Contract No. FA8650-04-C-1707, CDRL A001 between the USAF/AFMS AirForce Research Laboratory and General Atomics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to wireless sensor networks, andmore specifically to scheduling data transmissions and providing failednode recovery in a wireless sensor network.

2. Discussion of the Related Art

Wireless sensor networks have become popular in the recent years forforming a wireless network of various types of sensors that monitorconditions such as range, distance, temperature, sound, vibration,pressure, motion, or pollutants. Wireless sensor networks have beenimplemented in defense applications, environmental monitoring, habitatmonitoring, surveillance and security, industrial/commercial inventorytracking, process monitoring and disaster recovery. Some wireless sensornetworks are deployed as multi-hop “ad-hoc” networks. Typical ad-hocnetworks include network nodes that are deployed with little or noexisting infrastructure and form a network dynamically. These types ofnetworks, typically, have limited capabilities for adapting to a failednode. In addition, conventional sensor networks typically implementomni-directional antennas for communication by the various nodes.

Various aspects of wireless sensor networks, including medium accesscontrol (MAC), task scheduling, and networking protocols, have beenpreviously presented. Examples of such discussions can be found in U.S.Pat. Nos. 7,082,117, 7,075,476, 7,054,126, 6,986,161, 6,975,613,6,807,165, 6,754,188, and 6,208,247.

In addition, wireless sensors and wireless sensor networks are describedin various publications. Examples of such publications include variousUniversity of California Berkeley's projects such as Smart Dust, NEST,UCLA's NIMS and WINS projects, and the University of Florida's Atlasproject. Commercial companies producing various types of sensor networksinclude Sensoria, Dust networks, Ember Networks, Crossbow technologies,Pervasa and Sensicast Systems.

SUMMARY OF THE INVENTION

In one embodiment, the invention can be characterized as a sensornetwork having a first command center and a first access node,comprising a wireless transceiver, coupled to the command center. Thesensor network may also include a plurality of nodes individuallycomprising a wireless transceiver and a directional antenna, whereineach of the plurality of nodes is successively located in a downlinkdirection relative to the first access node, and is configured towirelessly communicate via the directional antenna with at least onenode of a first neighbor group in a first direction and at least onenode of a second neighbor group in a second direction. In addition, asensor device is individually coupled to at least one of the nodes, andis configured to provide sensor data for the first command center.

In another embodiment, the invention can be characterized as a methodfor scheduling data transmission in a wireless network having aplurality of nodes successively located in an uplink direction andproviding a communication link to an access node coupled to a commandcenter, wherein each given node of the plurality of nodes includes aneighbor group having at least two nodes located within an effectivewireless transmission range in the uplink direction of the given node.The method includes transmitting data from a transmitting node of theplurality of nodes to a receiving node in an associated neighbor groupof the plurality of neighbor groups during a first series of staggeredtime slots, and receiving the data at the receiving node during thefirst series of staggered time slots. The method further includesperforming the transmitting and the receiving operations for multipletransmitting nodes and a corresponding multiple receiving nodes of theplurality of nodes during the first series of staggered time slots.

In a further embodiment, the invention may be characterized as a methodfor node failure recovery in a wireless network having a plurality ofnodes successively located in a downlink direction relative to an accessnode coupled to a first command center, wherein each given node of theplurality of nodes includes a first neighbor group comprising nodeslocated within an effective wireless transmission range in the uplinkdirection of the given node and a second neighbor group comprising nodeslocated within an effective wireless transmission range in the downlinkdirection of the given node. The method includes detecting by aparticular node of the plurality of nodes that a recurring communicationhas not been received by the particular node, within a time period, froma neighbor node in either of the first neighbor group or the secondneighbor group. The method further includes communicating a firstindicator to the command center indicating that the neighbor node is afailed node, and transmitting an update neighbor information command toall nodes which are in the first neighbor group and the second neighborgroup of the failed neighbor node, causing such nodes to respectivelyupdate the first neighbor group and the second neighbor group to omitthe failed neighbor node.

In a still further embodiment, the invention can be characterized as asensor network including a plurality of nodes individually comprising awireless transceiver and a directional antenna, wherein each of theplurality of nodes is successively located in a downlink directionrelative to an access node, and is configured to wirelessly communicatevia the directional antenna with at least one node of a first neighborgroup in a first direction and at least one node of a second neighborgroup in a second direction. Each of the plurality of nodes is furtherconfigured to repeatedly transmit a signal using different segments ofthe directional antenna. The sensor network further includes a sensordevice individually coupled to at least one of the plurality of nodesand which is configured to provide sensor data, and a mobile nodeconfigured to be operable at varying geographical locations within anoperational range of the sensor network, wherein the mobile nodecomprises a wireless transceiver and is configured to establish wirelesscommunications with an associated node of the plurality of nodesresponsive to receiving the signal from the associated node.

In a still further embodiment, the invention can be characterized as amethod for discovering a mobile node operating in association with asensor network having a plurality of nodes individually comprising awireless transceiver and a directional antenna, wherein each of theplurality of nodes is successively located in a downlink directionrelative to an access node. The method includes wirelesslycommunicating, by each of the plurality of nodes, with at least one nodeof a first neighbor group in a first direction and at least one node ofa second neighbor group in a second direction. Further operationsinclude repeatedly transmitting, by each of the plurality of nodes, asignal using different segments of the directional antenna, receivingthe signal at a mobile node from an associated node of the plurality ofnodes, and establishing wireless communication between the mobile nodeand the associated node responsive to receiving of the signal from theassociated node.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of severalembodiments of the present invention will be more apparent from thefollowing more particular description thereof, presented in conjunctionwith the following drawings.

FIG. 1 is a diagram depicting a sensor network including various nodes,in accordance with an embodiment of the present invention.

FIG. 2 is a block diagram depicting a node according to variousembodiments of the present invention.

FIG. 3 depicts an example of assigning ordinals to nodes of the sensornetwork of FIG. 1, in accordance with an embodiment of the presentinvention.

FIG. 4 depicts network initialization operations in which a schedulingcommand is issued in accordance with an embodiment of the presentinvention.

FIG. 5 depicts a superframe which includes timeslots during whichvarious types of data may be transmitted within the sensor network ofFIG. 1, in accordance with an embodiment of the present invention.

FIG. 6 depicts one method for scheduling data transmission in the sensornetwork of FIG. 1, in accordance with an alternative embodiment of thepresent invention.

FIG. 7 depicts a parameter table which may be maintained by variousnodes of the sensor network of FIG. 1, in accordance with embodiments ofthe present invention.

FIG. 8 depicts an exemplary node transmission path and possible neighbornode tables in a non-failed node condition in accordance with anembodiment of the present invention.

FIG. 9 depicts an exemplary node transmission path and possible neighbornode tables in the presence of a failed node condition in accordancewith an embodiment of the present invention.

FIG. 10 depicts one method for scheduling data transmission during afailed node situation in the sensor network of FIG. 1, in accordancewith another alternative embodiment of the present invention.

FIG. 11 provides an example of network discovery of a mobile commandcenter, in accordance with still further embodiments of the presentinvention.

FIG. 12 is a flowchart depicting a method for discovering a mobile node,such as mobile command center, in accordance with an embodiment of thepresent invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

The following description is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles ofexemplary embodiments. The scope of the invention should be determinedwith reference to the claims.

FIG. 1 is a diagram depicting a sensor network including various nodesin accordance with an embodiment of the present invention. Inparticular, sensor network 100 includes command center 105 and firstaccess node 110, which is shown operatively coupled to the commandcenter. The sensor network may be configured with one or more nodes inwireless communication with the command center. In the embodiment ofFIG. 1, the sensor network is shown having a number of nodes (e.g.,depicted as node 0 through node 11) successively located in a downlinkdirection relative to command center 105 and first access node 110.Sensor network 100 is shown optionally configured with second accessnode 115 and associated second command center 120. As used herein, theterms “uplink” and “downlink” respectively refer to the direction ofcommunication toward and away from command center 105.

In one implementation, command center 105 may be realized using anysuitable device that provides one or more functions such as control,storing, monitoring, processing, and displaying of data generated orotherwise provided by various nodes of sensor network 100. For example,the command center may include a specific or general purpose computersuch as a personal computer, a laptop, a notebook, a personal digitalassistant (PDA), a hand-held computer or device, and the like. Thecommand center typically includes a user interface and display (e.g., anLCD, LED, CRT, plasma monitor, etc.) for displaying data provided by thenodes.

FIG. 1 is also shown having optional mobile command center 125, whichmay be configured with some or all of the functionality of commandcenter 105. A distinction is that the mobile command center is typicallyconfigured to couple to sensor network 100 at any of the nodes, not justvia an access node such as first and second access nodes 110, 115. Inaddition, the mobile command center, which is also referred to herein asa mobile node, can dynamically join and leave the sensor network. Mobilecommand centers are useful in situations in which some or all of nodes0-11 are installed in locations that are hard to access. If a nodefails, the mobile command center enables field personnel to easilyisolate failed or malfunctioning nodes without having to physicallyaccess such failed nodes.

Command center 105, which is typically operated by human operators,generally includes applications which communicate (e.g., send andreceive commands, data, and the like) with various nodes of the sensornetwork. Such applications may be implemented in software, hardware, andcombinations thereof.

The communication link operatively coupling command center 105 and firstaccess node 110 may be implemented using any suitable technique thatsupports the transfer of data and necessary signaling between thesecomponents. For example, the communication link may be implemented usingconventional wired and/or wireless communication technologies such asUSB, Ethernet, IEEE 1394, coaxial cables, serial or parallel cables, andoptical fiber, among others.

Command center 105 may be located in close physical proximity (e.g.,less than 1 meter) to first access node 110, but remote implementationsare also possible. Furthermore, in some embodiments, command center 105may be configured with network capabilities to support coupling withadditional command centers or other computers. In such embodiments, thecommand center may be configured with a network link that comprisesanything from a dedicated connection, to a local area network (LAN), toa wide area network (WAN), to a metropolitan area network (MAN), or evento the Internet. This arrangement may be implemented to permitcommunication between command center 105 and second command center 120.

First access node 110 is typically implemented to function as aterminating node that serves as a bridge between the nodes 0-11 andcommand center 105. A wireless transceiver may be implemented to supportcommunications with one or more nodes. The typical range of thetransceiver is such that a limited number of nodes are within theeffective communication range of the transceiver of the first accessnode. Several embodiments include first access node 110 operating todirectly communicate with only a terminating node (e.g., node 0) of thevarious sensor nodes. Redundancy may be achieved by permitting the firstaccess node to alternatively communicate with other downlink nodes inthe even that node 0 fails or malfunctions.

Each node of sensor network 100, such as nodes 0-11, generally includescapabilities to wirelessly communicate with one or more nodes of thesensor network. In an embodiment, each of the nodes 0-11 may besuccessively located in a downlink direction relative to first accessnode 110. Each node typically includes a wireless transceiver and adirectional antenna, and is configured to wirelessly communicate via thedirectional antenna with at least one node of a neighbor group in afirst direction (e.g., the uplink direction toward first access node110), and at least one node of a neighbor group in a second direction(e.g., the downlink direction away from first access node 110).

A neighbor group refers to one or more nodes with which a given nodecommunicates with in either the uplink direction or the downlinkdirection. For example, consider node 3, which may have a first neighborgroup, in the uplink direction, of nodes 1 and 2. Node 3 may thereforecommunicate with either or both of nodes 1 and 2 since such nodes are inits neighbor group. Similarly, node 3 may have a second neighbor group,in the downlink direction, of nodes 4 and 5. Node 3 is therefore alsopermitted to communicate with either or both of nodes 4 and 5 of thissecond neighbor group.

No particular number of nodes is required to be included in a neighborgroup. By way of non-limiting example, various embodiments will bedescribed herein which include 1, 2, or more, nodes in the neighborgroups. However, such teachings apply equally to neighbor groups ofother sizes. In some embodiments, a given node is configured tocommunicate directly with only those node or nodes in their associatedneighbor group. Using again node 3 as an example, node 3 will thereforeonly communicate with nodes 1 and 2 in the uplink direction and nodes 4and 5 in the downlink direction.

Generally, each node of sensor network 100 has the same sized neighborgroups, and the neighbor groups in the uplink and downlink direction arealso typically the same size. However this is not a requirement andneighbor groups with different numbers of nodes may alternatively beimplemented. One example includes the case of nodes 0 and 11, such thatnode 0 has a first or uplink neighbor group which includes first accessnode 110, and node 11 has a second or downlink neighbor group whichincludes second access node 115. Another example includes node 11omitting a downlink neighbor group, which is often the case when thesecond access node and associated command center 120 are notimplemented.

The various nodes 0-11 of the sensor system may use conventionalwireless technologies for communication such as, for example, IEEE802.11(g), IEEE 802.11(n), ultra-wideband (UWB), and the like. Onebenefit of UWB is that the underlying technology typically permits highdata rates over relatively short distances, as compared to traditionalnarrowband communication techniques. In addition, many UWB solutionshave relatively low power consumption, are cost effective, and can berobustly built with electronics that are easy to miniaturize usingASICs, for example. A typical UWB embodiment includes nodes whichimplement a wideband transceiver that transmits and receivescommunication bursts having a bandwidth of at least two, ten, or twenty,percent of a center frequency of the bursts.

In some embodiments, various nodes may include a directional antenna forcommunicating with the nodes of an associated neighbor group. A typicalimplementation includes use of segmented directional antenna patches forcommunication. The directional antenna has several advantages over anomni-directional antenna, for example. For instance, a directionalantenna permits the nodes to extend communication range, maintain agiven range with reduced power, and experience limited interference. Onereason for the limited interference is that each node does not directlycommunicate with all of the nodes of the sensor network. Instead, eachnode directly communicates with only those nodes which are included inan associated uplink or downlink neighbor group.

Each node, by virtue of its associated directional antenna, typicallycommunicates in only two directions; namely, the uplink direction towardfirst access node 110 and the downlink direction away from first accessnode 110. This arrangement may be accomplished using the directionalantenna, which may be segmented to communicate in a desired direction(e.g., in a direction which permits communication with a node of anassociated neighbor group). In this manner, the wireless sensor networkaccording to several embodiments represents a linear arrangement ofnodes, i.e., a linear sensor network. It is understood that as usedherein, the term linear does not necessarily mean linear as in astraight line (e.g., from node 0 to node 11), but that the connectionswithin a neighbor group are substantially in the linear direction of theuplink and downlink directions.

It is typically desirable for each of nodes 0-11 to know which antennasegment to use for communication with nodes located in the uplink anddownlink neighbor groups. In one example, the antenna of a particularnode may be configured or otherwise orientated during installation. Inanother example, a node may be configured during network set up to use aparticular segment. For instance, the nodes may obtain the antennasegment information by scanning to detect the neighbor nodes. Anotheralternative includes the case in which a node has location informationconcerning a particular neighbor node (e.g., GPS coordinates of thenode). In such a case, this location information may be used to selectthe appropriate antenna segment.

FIG. 2 is a block diagram depicting a node according to variousembodiments of the present invention. Node 200 may be used to implementsome or all of the nodes shown in FIG. 1, including nodes 0-11, thefirst and second access nodes, and mobile command center 125. Note thatin the case of the mobile command center, an omni-directional antenna isoften utilized instead of the directional antenna depicted in FIG. 2.

Node 200 is shown having RF system 202 coupled to digital system 204.The RF system is shown in communication with antenna 206, and power unit208 is shown providing power to the node. RF system 202 may includecomponents such as transmit and receive (TX/RX) switch 207 and sensor209, which is shown also including various communication componentsimplemented with an ASIC. The TX/RX switch functions with antenna 206.

In general, sensor 209 is a device configured to generate or otherwiseobtain data which can be stored at the associated node and/orcommunicated back to command center 105. Particular examples of suitablesensors include temperature sensors, light sensors, motion sensors,audio sensors, radar sensors, and the like. Sensor 209 may also beconfigured to monitor range, distance, vibration, pressure, orpollutants. In addition, the sensor may be implemented as a still orvideo camera capturing images which can be communicated to commandcenter 105. Low-power sensors are often implemented to maximize batterylife of the associated node. A typical embodiment includes one or moresensors individually coupled (e.g., internally or externally) to atleast one of the nodes 0-11 (FIG. 1).

Digital system 204 includes components such as processor 210, which isshown controlling digital ASIC 212 and data compression 214. Processor210 typically controls the overall operations of the node. Input andoutput from the digital system may be handled by as suitable componentsuch as digital I/O 216.

Memory 218 is generally used to store various types of data to supportthe processing, control, and storage requirements of node 200. Examplesof such data include program instructions for applications operating onthe node and data obtained from sensor 209. The memory may beimplemented using any type (or combination) of suitable volatile andnon-volatile memory or storage devices including random access memory(RAM), static random access memory (SRAM), electrically erasableprogrammable read-only memory (EEPROM), erasable programmable read-onlymemory (EPROM), programmable read-only memory (PROM), read-only memory(ROM), magnetic memory, flash memory, magnetic or optical disk,card-type memory, or other similar memory or data storage device.

Power unit 208 provides needed power to node 200 and may be implementedby battery 220. Power conditioner 222 provides any power conditioningthat may be desired. Antenna 206 is shown implemented as a directionalantenna such that it includes a patch array 224 and beam steeringcontrol 226.

Returning to FIG. 1, in accordance with various embodiments, each of thenodes 0-11 may provide one or more generalized functions such asobtaining sensor data using an associated sensor 208, communicating suchsensor data to a node in an uplink neighbor group, receiving sensor datafrom a node in a downlink neighbor group and relaying or repeating thissensor data to a node in the uplink neighbor group, communicatingcontrol data to nodes in both the uplink and downlink neighbor groups,and the like.

As noted above, some nodes may not include an associated sensor. Suchnodes therefore function to relay or repeat sensor data provided bydownlink nodes to a node in its associated uplink neighbor group.Accordingly, when a sensor is triggered based on a threshold condition,raw and/or processed sensor data may be communicated to command center105 via uplink communications from an originating node.

The nodes 0-11 of sensor network 100 may be positioned in variousconfigurations to meet desired application requirements. Typicalexamples include use of the sensor network in surveillance applicationsthat may be implemented in various types of monitoring scenarios inindustrial, military, security, and residential applications as well.

One embodiment includes positioning the nodes in fixed locations atdistances on the order of 20-80 meters. The distance between nodes maybe the same, or they may be positioned at different distances relativeto each other. In addition, such nodes may also be positioned atdifferent elevations to accommodate terrain changes, for example.

In addition, sensor network 100 may be implemented using most any numberof nodes to meet the needs of a particular application. The number ofnodes may range from as few as one or two nodes, to as many as a 1,000nodes, or more. For clarity and ease of discussion, the sensor network100 is shown having 12 nodes, referred to as nodes 0-11. An embodimentincludes configuring the sensor network using a linear arrangement ofnodes to cover a certain geographical region. Such an embodiment permitsmonitoring, via easily deployable sensor nodes, in diverse environmentand weather conditions.

Operation of sensor network 100 in accordance with various embodimentsof the present invention will now be described using, for example, thefollowing exemplary parameters. In particular, such parameters include:(i) the nodes are nominally spaced at effectively equal distance; (ii)the communication range of each node is two hops, that is, a node cancommunicate with each of its two immediate or adjacent neighbor nodes inboth the uplink and downlink directions; (iii) the interference range isfour hops, that is, a node may receive interference from nodes up tofour hops away; (iv) nodes may transmit, receive, relay, andcombinations thereof data (e.g., control data, sensor data, status data,etc.); (v) each node has a set of segmented directional antenna patchesand each antenna segment can be individually operated in order tomaximize communication range; and (vi) one or more nodes may include anantenna that can be used in a quadrant mode (e.g., antenna broken intofour quadrant segments) to support mobile command center discovery. Itis understood that other embodiments may have different parameters, thatis, other embodiments may include one or more of the parameters aboveplus one or more additional parameters.

Prior to actual operations, sensor network 100 will typically undergopreliminary processes such as synchronization and initialization. Forexample, during a typical synchronization process, first access node 110may be configured to start the network, causing each of the remainingnodes 0-11 to be configured with the MAC address of a neighboring nodeto which it is to synchronize. This information may be carried indownlink synchronization packets such that each node obtains itsneighbor address from the received synchronization packet. In addition,the synchronization packet may include other types of data such as, forexample, the node identification (ID) of its uplink neighbor node.

Each of the nodes 0-11 may also be configured with a secondary MACaddress (used as an alternate in the same direction as the node with theprimary MAC address). This secondary MAC address is often utilized insituations in which the primary node fails. For example, consider thesituation in which node 2 utilizes node 1 as its primary synchronizationsource and node 0 as its secondary synchronization source. Each of theother nodes of the sensor network may implement similar synchronizationtechniques. Toward the end of the network, last node 11 will typicallyrecognize that it is the terminating node and consequently sends a startmessage to the uplink node 10 so that operations may start. Note theexample of FIG. 1 shows last node 11 as being in communication withoptional second command center 120 via second access node 115.

With regard to synchronization during network creation, first accessnode 110 starts the network by issuing a beacon as a downlinkcommunication to adjacent node 0 on a periodic (e.g., every 500 ms) orother basis. This synchronization is sequential to the extent that eachnode 0-11 will synchronize in succession beginning with access node 110.For instance, after access node 110 is configured, successive node 0will then be configured such that it will listen for the beacontransmitted by access node 110. Upon receiving the beacon, node 0 willsynchronize to the network by setting its clock, for example, to thetimestamp present within the beacon. Node 0 may also adjust for otherfactors such as the known fixed receive processing and propagationdelays.

Once node 0 is configured, it will transmit a beacon to its downlinkneighbor node 1. This process is typically repeated until all of thenodes of sensor network 100 have been configured. In this example, eachnode will synchronize to its previously installed uplink neighbor node(e.g., its primary synchronization source node) which has beenpreviously synchronized.

With regard to synchronization in an established network, during asteady state, beacons may be transmitted in various uplink and downlinkcommunication time slots (e.g., time slots of a superframe) at aperiodic (e.g., 500 ms) or other rate. An exemplary superframe inaccordance with various embodiments will be described with regard to alater figure.

Packets transmitted in superframe time slots often include anidentification of the transmission slot utilized to transmit the packet.This feature, coupled with local timestamps of when packets are receivedat a particular node, may be used to adjust time slot timing to assistin maintaining node synchronization. According to one embodiment, toavoid or minimize oscillation, node timing may be achieved by aligningto downlink neighbor nodes only. According to an alternative embodiment,if the sensing and communication operations do not cause mutualinterference when operated simultaneously, then the scheduling of theactivity at the node may be simplified.

Network Initialization

FIG. 3 depicts an example of assigning ordinals to nodes of sensornetwork 100 in accordance with an embodiment of the present invention.As one example, during network initialization, each node is assigned anordinal number corresponding to its relative location in sensor network100. Such ordinals may be used by the MAC address scheduler to determinethe order of operation of the various nodes for sensing andcommunication functions. During this procedure, command center 105transmits an initialization (INIT) command frame, via first access node110, to first node 0. Node 0 will receive the transmitted INIT command,causing the MAC, for example, of node 0 to set its ordinal value to anORDINAL field of the INIT command. In this example, node 0 is assignedordinal 0. The MAC of node 0 may then increment the value in the ORDINALfield (ordinal=1) and then retransmits the INIT command to the downlinkneighbor node 1 which is then assigned ordinal 1.

This procedure is successively performed by each node of the sensornetwork resulting in the assignment of the ordinal values of all nodes0-11. The last node 11 is shown transmitting the INIT command, viasecond access node 115, to the downlink second command center 120.

FIG. 4 depicts network initialization operations in which a schedulingcommand is issued in accordance with an embodiment of the presentinvention. This operation typically follows the initializing operationdepicted in FIG. 3. In particular, consider FIG. 4 when second commandcenter 120 receives this INIT command. In this scenario, the secondcommand center responsively issues a RESUME-SCHEDULING command, which iscommunicated via the appropriate access nodes to the last node in thesensor network.

In FIG. 4, node 11 is the last node and receives the RESUME-SCHEDULINGcommand from second access node 115. The RESUME-SCHEDULING command isshown successively retransmitted to each uplink neighbor until it isreceived by command center 105. After command center 105 has receivedthe RESUME-SCHEDULING command frame, its MAC may then initiate normalscheduling and communication operations.

As noted above, certain embodiments do not include second command center120 and associated second access node 115. In such embodiments, theRESUME-SCHEDULING command is alternatively generated by the last node ofthe sensor network, which in the case of FIG. 4 is node 11.

In accordance with some embodiments, probe transmission may be part ofnetwork initialization to allow for automatic discovery of neighboringnodes and the antenna sector segment to be used for communication. Theprobe transmission aspect may be implemented instead of using a staticconfiguration in a node parameter table, for example.

In a typical probe transmission scenario, each node in turn sends asequence of preambles on each antenna sector while the other nodeslisten for such communications. Sending only preambles is oftenimplemented to optimize scanning time. Based on these preambles, thelistening nodes can determine their neighbors, which is a useful featurefor later operations such as scheduling. Neighbors of a given nodeinclude those nodes which are in the single hop and multiple hop rangein both the uplink and downlink directions. Put another way, the probingoperation permits each node to determine the nodes which are to beincluded in its uplink and downlink neighbor groups.

The probing operation typically requires that the nodes know theschedule of transmission; that is, the order in which each node willperform the probing operation. Accordingly, probing operations typicallyfollow the initial operation of ordinal assignments, as discussed above.One alternative includes configuring each of the nodes to transmitframes which include their respective IDs, thus allowing the nodes intheir interference range to be computed from the received frames.

For reasons that will become clear, various scheduling techniquespresented herein utilize information relating to a given node's set ofinterference nodes (i.e., the set of nodes in the uplink and downlinkdirection which would receive a signal transmitted from a given node).In certain cases, the interference set may be statically assigned to bethe 2-hop or 4-hop, for example, uplink or downlink neighbors.

An alternative technique for determining the interference set includeseach node to in turn transmit a signal, such as a preamble. The othernodes in the sensor network then listen during this transmission timeand record, detect, or otherwise determine if the signal has beenreceived. In addition, each of the listening nodes may compare thesignal with a threshold above which interference is deemed to occur.Other methods may adapt the interference dynamically by adjusting thethreshold depending on operational error rates, for example.

Regardless of the technique utilized, this process is repeated for eachnode in sensor network 100. After the last node has sent its signal,every node knows its respective interference set. If desired, theinterference set may be exchanged by configuring nodes 0-11 to send thisinformation to either or both of the command centers 105, 120. If thereception information is sent in both the uplink and downlink directions(e.g., sent to both command centers 105, 120), then all of the nodeswill have received the interference set of all of the other nodes.According to these examples, nodes 0-11 may compute their interferencesets, and thus their respective schedules, based upon the measuredinterference set instead of a statically assigned interference set.

FIG. 5 depicts a superframe which includes timeslots during whichvarious types of data may be transmitted within the sensor network inaccordance with an embodiment of the present invention. According tothis embodiment, the scheduling of data transmission is based upon atime division multiple access (TDMA) scheme. Superframe 500 is shownhaving a duration of 500 ms, and consisting of 200 slots (slots 0-199)of 2.5 ms each.

In an embodiment, slots 0-11 include assorted control data which may beused to support network operations. In particular, slot group 505includes slot 0 for even ordinal nodes and slot 1 for odd ordinal nodes.This arrangement reduces the possibility of interference betweenadjacent nodes during sensing operations.

Slot group 510 is reserved for mobile node discovery operations suchthat slot 2 is for receiving and slot 3 is for transmission. In general,mobile command center nodes have reduced operational ranges since theyare often implemented using omni-directional antennas (although this isnot a requirement). In many cases, mobile command center 125 is a nodewhich communicates with a limited number of nodes 0-11 (FIG. 1) and, insome cases, communicate with only a single node at any given time.

For example, mobile command center 125 may transmit its preambles inslot 2 while the relevant node, of the fixed nodes 0-11, may beconfigured to determine the antenna segment to receive transmissionsfrom the mobile command center. Slot 3 relates to fixed node preambletransmission, which may therefore be detected by a mobile command centerscanning for such transmissions.

Slot group 515 includes four slots reserved for control traffic(including beacons) in the uplink direction. Since there is generally nophysical layer broadcasting because of the use of directional antennasof the various nodes 0-11, control traffic may be sent to 1-hopneighbors so that each node in the sensor network receives the controltraffic. In slot group 515, the slots are used by nodes with ordinal {x,x+1, x+2, x+3} respectively for values of x from 0 to the highestinteger multiple of 4 in the ordinal set.

Slot group 520 includes four slots which are reserved for controltraffic (including beacons) in the downlink direction. In this slotgroup, the slots are used by nodes with ordinal {x, x+1, x+2, x+3}respectively for values of x from 0 to the highest integer multiple of 4in the ordinal set.

It is generally understood that capture effect includes the suppressionof a weaker of two signals received at the receiver. Accordingly, whenboth signals are nearly equal in strength, or are fading independently,the receiver may switch from one signal to the other signal resulting inthe phenomena known as picket fencing. If capture effect is not used, anadditional two slots may be implemented to accommodate the additionallyreceived signal. In such a scenario, slots 12 and 13 may be implementedfor such information and data traffic may be included in slots 14-199.Otherwise, if capture effect is used, then slots 12-199 are availablefor data transmission by way of data slot group 525. For simplifying thesystem, no optimization may be applied to the control traffic to andfrom the first two and the last two nodes of superframe 500.

If data is not currently being transmitted, then control traffic may berepeated in the unused portions of the superframe (i.e., the slotsnormally used to transmit data) by repeating the control trafficschedule of slots 2-11, or slots 2-13 without capture effect.

Data frames, which are typically used to transfer data generated by thevarious nodes 0-11 (e.g., sensor data), may be exchanged by exploitingthe concept of spatial reuse in slots 12-199, or slots 14-199 withoutcapture effect.

In some embodiments, data transfer is responsive to a SEND SENSOR DATAcommand provided by command center 105. This command frame is typicallyused by the node to identify the direction (e.g., uplink or downlink)and data source node by ordinal number so that each node may know thetransfer direction and be able to compute the appropriate transmissionslots. As one example, data transmission occurs in either the uplink ordownlink direction in alternating slots where: the source node (x) andall nodes in the set {x−4*(1+n)} (where n=0, 1, 2 . . . m and x−2−4*m isthe largest integer smaller than the ordinal of the uplink commandcentre node (node ‘0’)) transmit in the target direction in even slots.In odd slots, node x−2 and nodes in the set {(x−2}−4*(1+n)} transmit inthe target direction.

Scheduling

FIG. 6 depicts one method for scheduling data transmission in a wirelessnetwork, such as sensor network 100 of FIG. 1, in accordance with analternative embodiment of the present invention. This schedulingtechnique will be described with occasional reference to the sensornetwork and associated components depicted in FIG. 1, but it isunderstood that the disclosed technique is not limited to the depictednetwork, or any other network. It is further understood that the sensornetwork includes a plurality of nodes 0-11 successively located in anuplink direction and which provide a communication link to first accessnode 110. In addition, each given node of nodes 0-11 include a neighborgroup having at least two nodes located within an effective wirelesstransmission range in the uplink direction of the given node.

In FIG. 6, slots 12-18 refer to the data transmission time slots ofsuperframe 500, which is depicted in FIG. 5. Each slot 12-18 is shownwith nodes 0-9, and the arrows reflect a data transmission between twonodes. This example therefore depicts uplink communication of data fromnode 9. For example, at the top-most slot 12, the arrow from node 9 tonode 7 indicates that node 9 transmits data to node 7 during time slot12. With this understanding, a data transmission and schedulingtechnique will now be described.

At a given time, command center 105 may instruct node 9 to transmitdata. At this point node 9 (x=9) begins the process to transmit itssensor data to node 0 for uploading to command center 105. The first set{x, x−4*(1+n)} yields {9, 5, 1}. The second set {(x−2), (x−2)−4*(1+n)}yields {7, 3}.

In this example, slot occupancy is as follows:

Slot 12: Node 9

Node 7 (Nodes 5 and 1 have no data)

Slot 13: Node 7

Node 5 (Node 3 has no data)

Slot 14: Node 9

Node 7; Node 5

Node 3 (Node 1 has no data)

Slot 15: Node 7

Node 5; Node 3

Node 1

Slot 16: Node 9

Node 7; Node 5

Node 3; Node 1

Node 0

Slot 17: Node 7

Node 5; Node 3

Node 1

Slot 18: Node 9

Node 7; Node 5

Node 3; Node 1

Node 0.

The slot schedule for slots 17 and 18 repeats until the sensor data(e.g., an image) is transferred to the first access node and ultimatelycommand center 105.

For transfers in the downlink direction, the sets are {x, x+4*(1+n)} and{(x+2), (x+2)+4*(1+n)} for n=0 . . . m, x+2+4m being the smallestinteger larger than the ordinal of the downlink command center 120.

In accordance with further embodiments, the forgoing may be furthergeneralized as follows. One operation includes transmitting data from atransmitting node (e.g., node 9) to a receiving node (e.g., node 7) thatis in an associated neighbor group of the transmitting node. Thistransmitting operation may occur during a first series of staggered timeslots, such as during time slots 12, 14, 16, and 18.

Another operation includes receiving the data at the receiving nodeduring the first series of staggered time slots. A further operationincludes performing the forgoing transmitting and receiving operationsfor multiple transmitting nodes (e.g., nodes 5 and 1), and acorresponding multiple receiving nodes (e.g., nodes 3 and 0) during thefirst series of staggered time slots.

Various optional operations may be implemented in accordance withalternative embodiments of the present invention, and such operationswill now be described. For instance, one optional operation includestransmitting data from a transmitting node (e.g., node 7) to a receivingnode (e.g., node 5) in an associated neighbor group during a secondseries of staggered time slots, and then receiving the data at thereceiving node during this second series of staggered time slots. Thefirst and second series of staggered time slots are shown as beinginterlaced.

FIG. 6 makes clear that the receiving node in a previous time slot(e.g., node 7 in time slot 12) of the first series of staggered timeslots is the transmitting node in a later time slot (e.g., node 7 intime slot 13) of the second series of staggered time slots.

If desired, the transmitting and receiving may be performed for multipletransmitting nodes, and a set of corresponding multiple receiving nodes,during the second series of staggered time slots.

In addition, procedures may be implemented to account for a failed ormalfunctioning node. In such a case, the forgoing receiving andtransmitting is repeated unless an expected receiving node of theassociated neighbor group is identified as a failed node. In such ascase, the received data is instead transmitted to an alternativereceiving node of the associated neighbor group during a time slot ofthe first series of time slots. Consequently, the alternative receivingnode receives such data during the time slot of the first series of timeslots. Various techniques for handling node failure will be described inmore detail with regard to later figures.

FIG. 7 depicts a parameter table which may be maintained (e.g., in thememory 218) by various nodes of the sensor network in accordance withembodiments of the present invention. In some cases, such parameters andother related data are useful to the functioning of the associated node.With regards to network communications, each node may include datarelating to uplink and downlink neighbor nodes. The FIG. 7 exampleindicates that a given node includes a total of four neighbors, twouplink neighbors and two downlink neighbors. The depicted parametertable may therefore be increased or decreased to reflect the actualnumber of neighbors for a given node.

Data group 700 refers to data relating to a first neighbor node, and mayinclude: (i) the MAC address (of the first neighbor node); (ii) thecurrent node antenna sector to be used to communicate with the firstneighbor node); (iii) link direction (uplink/downlink); (iv) hop status(one hop or two hops); (v) location coordinates of the first neighbornode (e.g., GPS coordinates). Data relating to the other neighbor nodesmay be similarly included in the parameter table, and are depicted asdata groups 705, 710, and 715.

The data depicted in FIG. 7 may be populated before or after networksetup using any of a variety of different techniques including manualinput, mobile configuration nodes, command frames provided by thecommand controller, and the like.

Failure Recovery

Since sensor network 100 utilizes wireless communications, which areprone to failure and malfunction, various recovery mechanisms may beimplemented to handle link and node failures, for example. Severalexamples of such recovery mechanisms in accordance with variousembodiments will now be described.

In one example, failure to receive a beacon frame from a neighbor nodeduring m superframes may be interpreted as a node failure of theneighbor node. In this example, the detecting node may send a commandframe, such as a neighbor fail notification command frame, to thecommand center closest to the failed node (e.g., command center 105). Ifthe necessary communication link is broken as a result of the failednode, then the command frame may be sent to an alternate command center,such as second command center 120. Note that in some cases the failednode will be located further down the downlink direction such thatsecond command center 120 will be the closest command center and thealternate command center will be command center 105.

If desired, action can be taken to replace the failed node and perform anew node insertion operation, or the ordinal sequence can bere-established using a single uplink or downlink neighbor node. Nodeswith a single uplink or downlink neighbor node would therefore use thesame neighbor for both command and data transfers.

During typical operations, each node transmits commands to 1-hopneighbor nodes and data to 2-hop neighbor nodes. However, the receipt ofa neighbor fail notification command sent by a detecting node causescommand center 105 to send a command, such as anUPDATE-NEIGHBOR-INFORMATION command, to each of the uplink and downlinkneighbor nodes of the failed node. The receiving nodes consequentlyupdate their configuration data (e.g., parameters depicted in FIG. 7),to remove the failed node. In some cases, the updated nodes may send amessage, such as an UPDATED-NEIGHBOR-TABLE notification, to commandcenter 105.

Responsive to receiving this notification, the command center may issuea command, such as an initialization (INIT) command to the various nodes0-11 (exclusive of the failed node) to re-establish the ordinal sequenceof the nodes. Accordingly, in some cases, otherwise normal operation maythen be resumed after command center 105 receives the RESUME SCHEDULINGcommand sent by second command center 120, or by last node 11 when thesecond command center is not present.

Failed Node Scheduling

Consider now scheduling in circumstances where the sensor networkincludes a failed node. Reference is first made to FIG. 8, which depictsan exemplary node transmission path and possible neighbor node tables ina non-failed node condition in accordance with an embodiment of thepresent invention. If the ordinal number of the originating or initiallytransmitting node is even, a normally operating sensor network with allnodes present (i.e., each node having functioning 1-hop and 2-hopneighbor nodes) will transfer each packet along the even numbered 2-hopportion of the network until reaching the last node, or its predecessorin the special case of an end condition that may modify normal routing.Similarly, if the originating node is an odd number node, the nodetransmission path will be along the odd half of the chain of nodes.

FIG. 8 further shows sensor network 100 having nodes 0-9 (nodes 10 and11 have been omitted for clarity). In this example, node 9 is theoriginating node that initially sends data obtained from an associatedsensor, for example. Node tables 805, 810, 815, and 820 are shown fornodes 1, 2, 3, 4, and 5, respectively. Each node table identifies 1-hopand 2-hop neighbor nodes in both the uplink and downlink directions. Afailed node condition will now be considered.

FIG. 9 depicts an exemplary node transmission path and possible neighbornode tables in the presence of a failed node condition in accordancewith an embodiment of the present invention. In this example, node 3 isshown as the failed node.

Note that the various nodes tables of FIG. 9 are similar in manyrespects to the node table of FIG. 8. The primary distinctions relate tothe updating of the node tables to reflect the failed node 3. Inparticular, FIG. 9 does not include a node table for node 3 since thisnode has failed. In addition, node tables 800, 805, 815, and 820 havebeen updated to reflect that node 3 has failed. For instance, in node 1,node table 800 now refers to node 2 for both the 1-hop and 2-hopneighbors. In addition, for node 2, node table 805 refers to node 4 forboth the 1-hop and 2-hop neighbors. Node tables 815 and 820 aresimilarly updated. An exemplary scheduling technique using the precedingfailed node 3 scenario will now be described with regard to FIG. 10.

FIG. 10 depicts one method for scheduling data transmission during afailed node situation in a wireless network, such as sensor network 100of FIG. 1, in accordance with an another alternative embodiment of thepresent invention. This scheduling technique is similar in many respectsto the technique shown in FIG. 6. Distinctions between these techniquesrelate primarily to handling of the failed node condition, which is thefailure of node 3 in the exemplary case of FIG. 10.

In FIG. 10 slot 13, node 5 is shown receiving the data frame from node7. Ordinarily (i.e., in non-failed node conditions), node 5 would relayor otherwise transmit the received data to node 3. However, since node 3is a failed node, various failed node operations are performed. Inparticular, when node 5 receives this data, it will in turn transmit thedata to the uplink node 4, which is consistent with the node entry innode tale 820 (FIG. 9).

Note that any node with a double entry in the node table (e.g., node 4being the 1-hop and the 2-hop neighbor node in both the uplink anddownlink directions of node table 820), should listen on both the oddand even schedule slots in the expected direction. With this change,node 4 will listen when node 5 transmits on the odd node schedule andreceives the data frame instead of node 3. Node 4 should also transmiton the odd schedule since the originator node was odd and the even nodeswill be saving power on the even slots, except those nodes with doubleentry neighbors as noted above.

Node 2 upon receiving the data frame will typically check to identifythe originator node ordinal to determine whether is should transmit onthe even or odd half of the chain (and hence resulting in a cross over).The special case in this example is where node 2 could send directly tonode 0, if so desired.

Node 2, recognizing that node 9 is the originator node, transmits theframe on the odd schedule where node 1 will be listening. From thispoint, normal (i.e., non-failed node) scheduling is performed.

In accordance with some embodiments, a more generalized method for nodefailure recovery in a wireless network having a plurality of nodes(e.g., nodes 0-9 of FIG. 10) successively located in a downlinkdirection relative to an access node will now be described. It is againunderstood that each given node of nodes 0-9 include a first neighborgroup having nodes located within an effective wireless transmissionrange in the uplink direction of the given node and a second neighborgroup having nodes located within an effective wireless transmissionrange in the downlink direction of the given node. Examples of suchneighbor groups are illustrated by the various node parameter tablesshown in FIGS. 8 and 9.

One operation includes detecting by a particular node (e.g., node 4)that a recurring communication, such as a periodic beacon, has not beenreceived within a time period from a neighbor node (e.g., node 3). Againthe neighbor node may be in either the uplink neighbor group or thedownlink neighbor group.

Another operation includes communicating a first indicator to anappropriate entity, such as command center 105, for example, indicatingthat the neighbor node is a failed node. This indicator may beimplemented as a neighbor fail notification command frame.

A further operation includes transmitting a command, such as an updateneighbor information command, to all nodes in the first (uplink)neighbor group and the second (downlink) neighbor group of the failedneighbor node, causing such nodes to respectively update their nodetable to the extent that their uplink and downlink neighbor groups omitthe failed neighbor node (e.g., node 3).

In some cases, some or all of the updated nodes may communicate anupdate notification to command center 105 notifying the completion ofthe update of an associated node table to reflect omission of the failednode 3.

If resequencing is desired, then another operation may includetransmitting an initialization command successively in the downlinkdirection to each of nodes 0-2 and 4-9. In an embodiment, theinitialization command reestablishes a relative sequence of the nodes.

In an embodiment, data frames that have crossed from the odd ordinalchain to the even, and vice versa, may be readily detected by thefollowing method: (i) logical AND the frame source ordinal and the localnode ordinal; and (ii) test the least significant bit (LSB)—if the LSBis non-zero then the frame has crossed. With regard to frame loss errorrecovery, retransmission using the foregoing spatial reuse techniquesmay also be implemented.

There are a number of failure cases where error recovery may beimplemented. One case relates to node failure as described above.Another case is referred to as a loss of commands. It is first notedthat each time slot of the superframe, including the command time slots,may include a duration of 2.5 ms. This duration is sufficient totransmit eight command frames during each time slot. Several strategiesfor recovering from loss of commands will now be discussed.

A first strategy includes configuring the nodes to acknowledgesuccessful receipt of a command in a subsequent uplink or downlinktransmission time slot. Failed command transmissions can therefore berepeated in a later command time slot in the event that a particularcommand is not acknowledged. The ability to transmit multiple commandframes in each command slot provides sufficient capacity for anynecessary retransmissions of the commands.

A second strategy includes repeating each command multiple times in agiven time slot or within subsequent slots (e.g., in unused data timeslots). The probability of command loss can therefore be reduced to asurprisingly low level by repeating these commands. A third strategyincludes implementing a combination of the just-described first andsecond strategies.

Another failure case relates to data loss. For instance, failure tocorrectly receive a data frame typically interrupts the sequence offrames used to transfer data, such as an image, generated by anoriginating node. Since the data frames are commonly communicated insequence (e.g., uplink direction toward command center 105), each of thenodes typically includes memory for storing the image or other data thatis to be transmitted. Intermediate nodes, including relay nodes, willoptimally include memory to buffer frames during retransmissionattempts.

It is understood that with regard to the transmission of data frames,such data frames will include sequence numbers ordered starting with thelowest unacknowledged data frame. Any data frame not correctly receivedand not being within the sequence numbers of a subsequent transmissionsequence is implicitly discarded and will not be delivered.

Mobile Node Discovery

FIG. 11 provides an example of network discovery of a mobile commandcenter. In particular, such embodiments of the present invention relateto techniques facilitating network discovery of a mobile command center.Discovering a mobile command center poses interesting challenges insituations in which various fixed nodes 0-11 of sensor network 100utilize segmented directional antennas on the fixed nodes.

In one implementation, a mobile command center, such as mobile node 125,utilizes an omni-directional or quadrant (4 segments instead of a largernumber of segments as in a typical node) in order to improve nodediscovery time. Using segmented directional antenna patches on both thefixed nodes 0-11 and the mobile node could take significantly long timeperiods to establish communications because of the time necessary toalign the antenna segments of the node and mobile command center.Alternatively or additionally, the nodes may be configured to transmitpreambles on a periodic or other basis to facilitate detection of themobile command centers.

Further configurations include transmitting multiple messages with themessage number included in the message, and a combination oftransmitting preambles and transmitting multiple messages.

In one embodiment, mobile node 125 includes an antenna having a quadrantmode in which antenna gain can be 6 dB less than the gain in the defaultdirectional segment mode, while still transmitting at an allowedeffective isotropic radiated power (EIRP). One feature of thisconfiguration permits a larger azimuth of about 90 degrees withoutsacrificing range. The mobile node can also receive in this mode, butthe range me be somewhat reduced (e.g., reduced to about 40 meters).This mode may be used for mobile node discovery to reduce discoverytime.

Various configurations are possible in accordance with variousembodiments of the present invention. With regard to sectored antennas(quadrant mode), four antenna sectors may be merged into one quadrantfor communication. This arrangement results in five quadrants from thetypically available antenna sectors (e.g., 4-18 sectors).

Another case relates to the situation in which mobile node 125 hasknowledge of the location of itself and a fixed node, such as node 3.This location information may include GPS coordinates of these nodes. Inthis embodiment, the mobile command center can calculate the antennasectors (e.g., three or more sectors typically under optimalcircumstances) that the mobile node will use to communicate. Accordingto this arrangement, the worst case node discovery time may becalculated as follows:5*3*0.5 seconds=7.5 secondsWhere 5 represents the number of quadrants of the mobile node and 3 isthe number of antenna sectors.

Yet another case relates to automatic mobile node discovery during whicha fixed node, such as node 3, transmits beacons on each of itsoverlapping quadrants (e.g., 5 quadrants) in the mobile node discoverytransmit slot. Node 3 also looks for beacons from the mobile node in thesame quadrant in the mobile node discovery receive slot. This process ofdiscovery is typically once per superframe. The pattern may be repeatedon a periodic or other basis until the mobile node is detected or untila timeout threshold has been reached, for example.

In one embodiment, mobile node 125 dwells in each quadrant for fivesuperframe periods, and listens until it detects a beacon from atransmitting node such as node 3 as depicted in the figure. After themobile node receives the beacon, it synchronizes and then also transmitsa beacon. In the preferred embodiment of the invention, the worst casenode discovery time in this example is:5*5*0.5 seconds=12.5 seconds.Where one 5 relates to the number of quadrants of the fixed node and theother 5 relates to the number of superframe periods that the mobile nodedwells in a particular quadrant. This technique is relatively flexible,but is somewhat slower than other discovery methods.

Referring still to FIG. 11, a more generalized example of a mobile nodediscovery process will now be described. In this example, a sensornetwork, such as sensor network 100 (FIG. 1), can discover or otherwisebecome aware of the presence of mobile node 125. It is understood that aplurality of nodes (e.g., two or more of nodes 0-11) each include awireless transceiver and a directional antenna. Such nodes may beconfigured to wirelessly communicate via a respective directionalantenna with at least one node of a first neighbor group in a firstdirection and at least one node of a second neighbor group in a seconddirection. Such aspects have been described in detail with regard toFIG. 1, for example.

In accordance with further embodiments, some or all of nodes 0-11 may befurther configured to repeatedly transmit a beacon or other signal usingdifferent segments of their respective directional antennas. Theserepeated transmissions of a beacon signal may be accomplished by cyclingthrough each antenna segment (e.g., one or more transmissions persegment) on a periodic or regular basis.

Mobile node 125 is typically configured to be operable at varyinggeographical locations relative to various nodes of the sensor network,such that the mobile node is positionable within an operational range ofthe sensor network. Upon receiving a beacon signal transmission from anassociated node (e.g., node 3) the mobile node may operatively couple(e.g., establish wireless communications) with node 3. Typically, mobilenode 125 will not receive all of the beacon signals transmitted by node3, but rather will only receive beacon signals transmitted using asubset of all of the antenna segments (e.g., only 1-3 segments).

In an embodiment, the coupling between node 3 and the mobile node may beaccomplished by establishing wireless communications between thesedevices using known communication protocols. Typically, node 3 willcommunicate with the mobile node using the same antenna segment used totransmit the beacon signal that was received by the mobile node.

In some embodiments, the sensor network is capable of utilizinginformation relating to a current physical or geographical location ofmobile node 125. For example, the mobile node may communicate itslocation information to a particular command center (e.g., commandcenter 105), which in turn communicates this information to theappropriate node or nodes 0-11. Once the nodes have received thisinformation, then an intelligent decision may be made with regard towhich segment or segments that a particular node should transmit thebeacon signal to mobile node 125. In other words, since a node hasknowledge of the location of the mobile node, then the node will knowwhich segment of an associated directional antenna to use to transmitthe beacon signal to the mobile node.

FIG. 12 is a flowchart depicting a method for discovering a mobile node,such as mobile node 125, in accordance with an embodiment of the presentinvention. This method will be described with occasional reference tosensor network 100 and related components depicted in FIG. 1, but it isunderstood that the disclosed method is not limited to a sensor network,or any other system or network. In general, the mobile node operates inassociation with a sensor network (e.g., sensor network 100) having aplurality of nodes individually having a wireless transceiver and adirectional antenna.

Block 1200 includes wirelessly communicating, by each of the pluralityof nodes, with at least one node of a first neighbor group in a firstdirection and at least one node of a second neighbor group in a seconddirection. Using node 3 as an example, the first direction relates tothe uplink direction and the first neighbor group includes nodes 1 and2. Likewise, the second direction relates to the downlink direction andthe second neighbor group includes nodes 4 and 5.

Block 1205 refers to repeatedly transmitting, by each of the pluralityof nodes, a signal using different segments of the directional antenna.This signal is typically implemented as a beacon signal.

Block 1210 relates to receiving the signal at a mobile node (e.g.,mobile node 125) from an associated node (e.g., node 3) of the pluralityof nodes.

Block 1215 includes establishing wireless communication between themobile node and the associated node responsive to receiving of thesignal from the associated node.

Although these embodiments may be implemented using the exemplary seriesof operations described herein, additional or fewer operations may beperformed. Moreover, it is to be understood that the order of operationsshown and described is merely exemplary and that no single order ofoperation is required.

Various embodiments described herein may be implemented in acomputer-readable medium using, for example, computer software,hardware, or some combination thereof. For a hardware implementation,the embodiments described herein may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,other electronic units designed to perform the functions describedherein, or a selective combination thereof. In some cases, suchembodiments are implemented by a controller which may include some orall of the components associated with digital system 204 of node 200.Some ASIC designs may be implemented as a stand-alone packaged device,or embedded as a soft Intellectual Property (IP) core in a larger systemASIC.

For a software implementation, the embodiments described herein may beimplemented with separate software modules, such as procedures andfunctions, each of which perform one or more of the functions andoperations described herein. The software codes can be implemented witha software application written in any suitable programming language andmay be stored in memory, and executed by a controller or processor.

While the invention herein disclosed has been described by means ofspecific embodiments, examples and applications thereof, numerousmodifications and variations could be made thereto by those skilled inthe art without departing from the scope of the invention set forth inthe claims.

1. A sensor network comprising: a first command center; a first accessnode coupled to the command center and comprising a wirelesstransceiver; a plurality of nodes individually comprising a wirelesstransceiver and a directional antenna, wherein each of the plurality ofnodes is successively located in a downlink direction relative to thefirst access node, and is configured to wirelessly communicate via thedirectional antenna with at least one node of a first neighbor group ina downlink direction and at least one node of a second neighbor group inan uplink direction; and a sensor device individually coupled to atleast one of the plurality of nodes and configured to provide sensordata for the first command center; wherein for each given node of theplurality of nodes, the first neighbor group of the given node comprisestwo nodes of the plurality of nodes and which are located in the uplinkdirection relative to the given node, and the second neighbor group ofthe given node comprises two nodes of the plurality of nodes and whichare located in the downlink direction relative to the given node; andwherein each node of the plurality of nodes is configured to selectivelycommunicate sensor data provided by a non-adjacent downlink node, of thetwo nodes located in the downlink direction, to either a non-adjacentuplink node or an adjacent uplink node of the two nodes located in theuplink direction based upon operation criteria.
 2. The sensor network ofclaim 1, wherein each node of the plurality of nodes is configured tocommunicate the sensor data provided by the non-adjacent downlink node,of the two nodes located in the downlink direction, to the non-adjacentuplink node of the two nodes located in the uplink direction.
 3. Thesensor network of claim 1, wherein the operation criteria identifiesthat the non-adjacent uplink node is a failed node, thus permitting thecommunicating of the sensor data to the adjacent uplink node.
 4. Thesensor network of claim 1, wherein each node of the plurality of nodesis configured to selectively receive the sensor data provided by eitherthe non-adjacent downlink node or an adjacent downlink node of the twonodes located in the downlink direction based upon operation criteria.5. The sensor network of claim 4, wherein the operation criteriaidentifies that the non-adjacent downlink node is a failed node, thuspermitting the receiving of the sensor data from the adjacent downlinknode of the plurality of nodes.
 6. The sensor network of claim 1,wherein each node of the plurality of nodes that include the sensordevice is configured to communicate the sensor data to the first commandcenter via an uplink communication in the downlink direction with a nodeof the first neighbor group.
 7. The sensor network of claim 1, whereineach node of the plurality of nodes is configured to receive the sensordata provided by a downlink node of the second neighbor group, and isfurther configured to communicate the sensor data to an uplink node ofthe first neighbor group.
 8. The sensor network of claim 7, wherein thereceived sensor data provided by the downlink node is generated by asensor device coupled to a node located in the downlink direction fromthe at least one node of the second neighbor group.
 9. The sensornetwork of claim 1, wherein the directional antenna of at least one ofthe plurality of nodes comprises a plurality of segments, wherein afirst one of the plurality of segments permits communications in thedownlink direction and wherein a second one of the plurality of segmentspermits communications in the uplink direction.
 10. The sensor networkof claim 1 further comprising: a second command center; and a secondaccess node coupled to the second command center, wherein the secondcommand center and the second access node are located in the downlinkdirection relative to the first access node, and wherein the secondaccess node is located to wirelessly communicate with a terminating nodeof the plurality of nodes.
 11. The sensor network of claim 1 wherein thewireless transceiver of each of the plurality of nodes comprises awideband transceiver which transmits and receives communication burstshaving a bandwidth of at least two percent of a center frequency of thebursts.
 12. The sensor network of claim 1 wherein the wirelesstransceiver of each of the plurality of nodes comprises a widebandtransceiver which transmits and receives communication bursts having abandwidth of at least ten percent of a center frequency of the bursts.13. The sensor network of claim 1 wherein the wireless transceiver ofeach of the plurality of nodes comprises a wideband transceiver whichtransmits and receives communication bursts having a bandwidth of atleast twenty percent of a center frequency of the bursts.
 14. The sensornetwork of claim 1, further comprising: a plurality of sensor devicesindividually coupled with one of the plurality of nodes, the pluralityof sensor devices each providing sensor data for the first commandcenter.
 15. The sensor network of claim 1, wherein at least one of theplurality of nodes is coupled with a plurality of sensors, the pluralityof sensors each providing sensor data for the command center.
 16. Thesensor network of claim 1, wherein the sensor device of at least one ofthe plurality of nodes is a sensor selected from the group consistingof: a temperature sensor, a light sensor, a motion sensor, and a radarsensor.
 17. The sensor network of claim 1, wherein the first access nodeand the plurality of nodes define a linear network of wireless nodes.18. The sensor network of claim 1 wherein each of the plurality of nodesis in a fixed location.
 19. The sensor network of claim 1 wherein eachof the plurality of nodes is configured to wirelessly and linearlycommunicate via the directional antenna with the at least one node ofthe first neighbor group in the downlink direction and the at least onenode of the second neighbor group in the uplink direction.
 20. A sensornetwork comprising: a first command center; a first access node coupledto the command center and comprising a wireless transceiver; a pluralityof nodes individually comprising a wireless transceiver and adirectional antenna, each of the plurality of nodes being in a fixedlocation, wherein each of the plurality of nodes is successively locatedin a downlink direction relative to the first access node, and isconfigured to wirelessly communicate via the directional antenna with atleast one node of a first neighbor group in a first direction and atleast one node of a second neighbor group in a second direction; and asensor device individually coupled to at least one of the plurality ofnodes and configured to provide sensor data for the first commandcenter; wherein for each given node of the plurality of nodes, the firstneighbor group of the given node comprises two nodes of the plurality ofnodes and which are located in the first direction relative to the givennode, and the second neighbor group of the given node comprises twonodes of the plurality of nodes and which are located in the seconddirection relative to the given node; and wherein each node of theplurality of nodes is configured to selectively communicate sensor dataprovided by a non-adjacent node in the second direction, of the twonodes located in the second direction, to either a non-adjacent node inthe first direction or an adjacent node in the first direction of thetwo nodes located in the first direction based upon operation criteria.21. The sensor network of claim 20 wherein each of the plurality ofnodes is configured to wirelessly and linearly communicate via thedirectional antenna with the at least one node of the first neighborgroup in the first direction and the at least one node of the secondneighbor group in the second direction.